The invention relates to a method and apparatus for generating a representation of a half-tone image.
In a typical system, such as that embodied in our Crosfield Magnascan 645, a record medium mounted on a rotating cylinder is exposed to a plurality (usually six) of scanning beams arranged side by side. During scanning, the cylinder rotates and the set of scanning beams moves parallel to the axis of the cylinder so that the entire record medium is exposed. The record medium is electronically divided into a grid of dot cells of square form and for each quadrant of the dot cell the density of the colour in question at that point in the original image is determined and the scanning beams are caused to generate a portion of a dot whose size relative to the dot cell is determined in accordance with that colour density. In practice, the half-tone image will be represented by four colour separations representing the densities of cyan, magenta, yellow, and black in the original image.
Typically, each pixel of the original, scanned image corresponds to approximately one quadrant of a dot cell. This is generally satisfactory when the resolution is relatively low (ie. graphics resolution) for example 9.times.10.sup.4 to 12.times.10.sup.4 pixels per square inch. However, even with an image containing solely graphics information, the edge of a graphics feature can appear ragged due to the relatively large size of a quadrant of a dot cell. Further problems arise when it is desired to combine graphics information with high resolution information such as text. Typically, when an image is scanned at high resolution there will be between 81.times.10.sup.4 and 144.times.10.sup.4 pixels per square inch. The main problem arises at the junction between image portions at low and high resolution. In particular, ragged edges or stepping will occur.
U.S. Pat. No. 4,276,567 describes a method of generating a half-tone image at a single resolution but with the ability to modify the edge of the image to minimise the stepping problem. The disclosure in that specification does not consider the problem of combining areas of high and low resolution.
GB-A-2102240 describes a method for composing and recording a picture having a continuous gradation and a character having two value levels such as white and black. This method, in a similar way to the method described in the U.S. specification mentioned above, enables a "high resolution edge" to be provided around a graphics resolution feature of the image but provides a very crude method for generating high resolution information. In this example the high resolution information has just two levels, white or black. The apparatus involved causes the half-tone dot generator used when recording graphics resolution information to be bypassed when high resolution text information is to be recorded. This does not solve the problem of the junctions between edges of different resolution.